There are available fire control systems which are capable of tracking a moving target and outputting a set of signals indicative of the target's instantaneous position and velocity. These signals are used in fire control systems to provide lead to the aiming of a gun that fire projectiles. The assumption made in such fire control systems is that the motion of the target at any given moment is linear, such that the required lead is a linear function of the target's instantaneous velocity and the computed time of flight of the projectile. An example of this type of fire control system is described in U.S. Pat. No. 4,004,729, issued Jan. 25, 1977 to Rawicz et al.
Estimated roll angles and load factors of a maneuvering target aircraft may be manually inputted into a fire control computer of the fire control system described in U.S. Pat. No. 4,146,780, issued Mar. 27, 1979 to Sprey. In response to other signals received from conventional target tracking and ranging devices, the computer first calculates linearly projected future positions of the target aircraft. Corrections, which may be curvilinear, to these projected positions are then calculated from the inputed estimates of aircraft roll angle and load factor. An operator can selectively actuate manual controls to cause the fire control computer to combine this correction with the linearly calculated future positions. Control signals corresponding to these new intercept positions are transmitted to conventional gun laying equipment to cause the gun to be aimed at the intercept positions.
In the radar tracking system described in U.S. Pat. No. 4,179,696, issued Dec. 18, 1979 to Quesinberry et al, a nine state Kalman filter is provided with angle and range track error measurements in a coordinate system that is aligned with and normal to the antenna line of sight (LOS coordinate system). The Kalman filter converts these measurements to predict target position referenced to a stable coordinate system.
In their previous U.S. Pat. No. 4,402,251, the applicants disclose a fire control system in which the apparent area of the maneuvering target is monitored in order to generate a gun line offset (i.e. an offset from the line of sight between the gun and the target, to compensate for the motion of the target, equivalent to lead) which is a function of the target velocity and acceleration. When the line of sight angular rate crosses through zero, (minimum apparent area) a firing command is initiated. This system is not without utility, in particular for a class of targets where the product of target velocity and projectile time of flight is much less than the engagement range. However, the inventors have now realized that further improvements can additionally be made.
Accordingly, it is an object of the invention to provide an improved fire control system useful in generating continuous gun line offsets from the line of sight to the target so as to improve the aim of a gun engaging a maneuvering target.
All common land, sea and air vehicles are elongated in the direction of their travel and they travel along straight lines unless an acceleration having at least a component perpendicular to the direction of travel is experienced. In a land vehicle lateral acceleration is provided by turning a wheel with respect to the direction of travel. In an airplane, lateral acceleration is provided by a control surface; similarly in the case of a ship. In each case, note that the acceleration must have a component perpendicular to the direction of travel if a turn is to be effected.
Basic physics teaches that the motion of an object subjected to an constant acceleration perpendicular to its velocity is an arc of a circle if the magnitude of the velocity (speed) is constant. Accordingly, the applicants have realized that if one can determine the instantaneous velocity and perpendicular acceleration of a target, one can then calculate the arc of the circle defined thereby, and can adjust the gun line offset accordingly.
It is an object of the invention to provide an improved gun line offset system, which comprises means for predicting the nonlinear lead (offset) when engaging a maneuvering target, so that the projectile will intersect the target a projectile time of flight later.
It is the ultimate object of the invention to provide an improved fire control system which allows maneuvering targets to be hit with increased frequency.